1. Field of the Invention
The present invention generally relates to arrays of radiation sensitive elements used in imaging systems and the like. More particularly, the invention relates to a monolithic superheterodyne detector pixel usable in such an array to receive a radio frequency signal at millimeter wavelength and produce a video output signal.
2. Description of the Prior Art
Focal plane arrays used in radiation imaging systems systems frequently contain over 100,000 pixel elements. Typically, such systems detect radiation occurring in the visible and infrared regions. There has long been a interest in developing focal plane arrays usable at millimeter wavelengths because of the ability of energy in this band to penetrate fog, snow, moderate rain and many visibly opaque materials. Unfortunately, with the technology previously available, it was not practical to implement a system usable at these frequencies which contained a large number of pixel elements. Recently, however, some of the inventors of this invention have demonstrated that a monolithic detector pixel may be constructed capable of detecting 94 GHz energy with tangential signal sensitivity (TSS) of better than -50 dBm (decibels relative to a milliwatt).
Such pixels, which can be incorporated into a much larger array of similar pixels, were constructed having a slot antenna defined in a metallized ground plane of GaAs substrate. A microstrip circuit containing a Schottky-barrier detector diode was fabricated on the substrate face opposite the ground plane. The microstrip circuit was configured to provide an optimum impedance match between the diode and the slot antenna. As a result, it was not necessary to effect tuning to free space utilizing external impedance matching elements as had previously been required. Such external impedance matching elements are undesirable because they add significant overall size to each pixel. The findings concerning this pixel were fully reported in the following article which is incorporated herein by reference: H. G. Henry, R. G. Freitag, R. R. Shaller, and M. Cohn, "A GaAs Monolithic Array of Impedance Matched Antenna/Detector Pixels For A 94 GHz Imaging System," presented at the 1989 IEEE International Electron Devices Meeting, Washington, DC, Dec. 3-6, 1989, Technical Digest, pp. 181-184.
In many applications, a much greater sensitivity than that achievable with simple video detection is required. In a radiometric mode for which a detected video bandwidth of 1 MHz is desired, a reasonable FkTB (where: F is a channel noise contribution (in dB), k is the Boltzmann constant, T is absolute temperature, and B is desired video bandwidth) figure would be approximately -104 dBm. This is significantly less than the -50 dBm sensitivity achievable with the state-of-the-art 94 GHz pixel described above. In other words, more than 60 dB of preamplification gain would be required in the received signal prior to detection. At such high frequencies, however, it is difficult to fabricate monolithic amplifiers having sufficient yield or reliability.